The most striking aspects of the new head-trauma study of former NFL players published in JAMA Neurology are not the formal findings per se. In many ways, those merely quantify and validate suspicions that even casual fans likely hold. The rub is that 40 percent of the retired players that researchers in Texas examined have some form of cognitive impairment. That's a glass-half-something. The good news is that almost two-thirds of players in the study seem fine as they age. But 40 percent is still scary odds when we're talking brain function. We all recognized it was some proportion of ex-players; now we're beginning to get a rough idea of how many.

What's more striking was the number of participants in the study who had no idea their mood-related problems—sleep disorders, weight fluctuation, anxiety, lack of initiative, nervousness, inability to make decisions, irritability, low energy—most likely stemmed from brain trauma. Those are all indicators of depression, says Dr. John Hart Jr., the lead author of the study conducted by the Center for BrainHealth and the University of Texas Southwestern Medical Center. He tells me that three-quarters of the ex-players who came in with those symptoms did not associate their issues with depression, nor did they realize their condition was treatable.

"You've got to ask for symptoms of depression," Hart says. "Those guys kill themselves and they're not going to come in with a group of symptoms and say, 'I've got this problem.'"

Beginning in 2010 the researchers examined 34 ex-players from around north Texas. Their mean age was 62. Thirty-two of them had suffered at least one known concussion; one participant reported 13. Twenty of the men showed normal cognition through a battery of tests. Those who didn't were diagnosed with some degree of cognitive impairment or, in two cases, dementia. Eight men, or nearly a quarter of the participants, were diagnosed with depression.

Depression should scare the shit out of anyone who has been concussed, soldiers and athletes alike. Junior Seau was depressed and then he killed himself. Ray Easterling was depressed and then he killed himself. Dave Duerson was depressed and then he killed himself. Jovan Belcher was depressed and then he killed the mother of his child and then he killed himself.

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When I asked Daryl Johnston, the former Cowboys fullback and a participant in the cognition study, what stuck with him about the findings, he pointed to depression. Football's machismo makes the disease a taboo. But he also knows a guy who played golf with Seau a few days before his suicide and couldn't believe, in half a day with Seau, that he didn't know how sick he was.

"The guys who are suffering from depression had no idea," Johnston says. "To me, that's the beginning of the slippery slope. If this is something they weren't even aware of, what happens in 18 months?"

The study also found, for the first time, a correlation between abnormalities in the brain's connective white matter and cognitive impairment. But the roster of what we don't know is staggering. According to Hart, we don't know whether a few colossal blows to the head are worse than the cumulative effect of hundreds or thousands. (In other words, whether the occasional mauling a wide receiver takes trumps the repeated collisions a left tackle absorbs.) We don't know whether taking a shot in high school or college is worse than taking one later in life. We don't know whether being able to brace for a blow helps mitigate it. We don't know if co-variants such as smoking or high cholesterol might intensify the effects of head trauma.

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In short, we don't know, Hart says, when we watch football, "Are we watching a train wreck?"

Even if we are, the treatments are, at least, crawling out of the dark ages. So are the diagnoses. Says Johnston: "We hope that medicine keeps pace with the size and speed of the players. I think this is that natural progression we've seen with other injuries in the NFL. A torn ACL back in the '70s was career-ending. That's just the way the progression goes. Hopefully we're going to follow that path with this."